Two-step evolution of the quasiparticle band with doping in the two-dimensional t-t '-t ''-J model

We study the doping dependence of photoemission spectra for the t-t'-t "-J model by using the exact diagonalization technique and present a consistent theoretical analysis. Both calculations show that upon doping the enhancem ent of incoherent motion of holes due to the t' and t " terms accounts for the formation of the flat region around (pi, 0) in the quasiparticle disper sion at underdoped and optimally doped region, despite the absence of the f lat band at half filling. Our results are in excellent agreement with resen t photoemission experiments on Bi2Sr2Ca1Cu2O8+delta [Marshall et al., Phys. Rev. Lett. 76, 4841 (1996)] and Sr2CuO2Cl2 [Wells et al., Phys. Rev. Lett. 74, 964 (1995)].